I think that harmful effects of radiation in places like that matter mostly in the context of life expectancy and the well-being of individual organisms (that may be unlucky to e.g. ingest some particularly radiating matter and quickly die), but not the general growth of the population. Higher cancer rates may not matter much if the organisms that get cancer can reproduce before they die. Compared to other types of pollution and other human-related dangers to wildlife, some level of radiation isn't that harmful to nature.